The first signs of a nebula -- the tenuous, largely transparent gas cloud thrown out by a star that has exploded as a supernova -- have been detected around 1987A, the supernova that astronomers have been watching from the southern hemisphere since it showed up last February.

Astrophysicists already have detected in the nebula some of the many different kinds of atoms and molecules that stars create during their lifetimes. Because stars are giant thermonuclear fusion reactions, welding hydrogen atoms (the simplest of all elements) into heavier atoms, they are the source of virtually all matter other than hydrogen itself.

These heavier elements accumulate in the star until it explodes and scatters them in all directions. It is from supernovae nebulae that new stars and planets condense. In this sense, Earth and human beings are made of stardust.

The element barium already has been detected in the nebula, as has carbon monoxide, a compound that is believed to have formed near the cooler outer reaches of the expanding nebula.

As with all deep space chemical analyses, the chemicals themselves cannot be seen directly. However, each element emits electromagnetic radiation at a characteristic wavelength when it is heated and this radiation can be detected on Earth.

Discovery of the nebula was reported last week by Mark Phillips of the Cerro Tolo InterAmerican Observatory and I.J. Danziger of the European Southern Observatory at a conference on the supernova at George Mason University in Fairfax.

The supernova is of unusual interest because it is the closest to Earth since the invention of the telescope some 400 years ago. Although the star actually exploded 163,000 years ago, light from the event has only just reached Earth.

After some early confusion about which star actually blew up, astronomers at the meeting said it is now agreed that it was a blue supergiant called Sanduleak -69 202. The star was in the Large Magellanic Cloud, a galaxy that orbits our galaxy. George Sonneborn of the Goddard Space Flight Center in Greenbelt said current observations show Sanduleak "has most certainly disappeared."